A residual gas analyzer of this kind is known as including: an ionizer; a mass spectrometer; a sensor unit provided with a sensor having a detector and an AC generator unit; and a device main body connected by a cable with respect to the sensor unit (ex. see non-patent document 1).
According to this residual gas analyzer, residual gas introduced into the ionizer is initially influenced by a thermal electron discharged from a filament at high temperatures and ionized. Generated ions are focused and accelerated by a lens, and introduced to the mass spectrometer. In the mass spectrometer, direct and alternating current voltages are applied to cylindrical electrodes of four (quadrupole) so that ions are sieved. Separated ions are detected as a current by a Faraday cup in the detector. This ion current is changed according to an amount of residual gas (partial pressure), so that the residual gas can be accurately measured.
However, as the ambient pressure in a vacuum chamber increases, the mean free path decreases and therefore the probability of ion-gas collisions in the quadrupole increases hence inhibiting the ion from reaching the detector. Therefore ion collisions inside the mass spectrometer will cause a drop in the sensor sensitivity. Such a drop increases with the ambient pressure. Therefore, when an ambient pressure in which the quadrupole is arranged exceeds a predetermined pressure (ex. about 1×10−2 to 1×10−1 Pa), an ion current reaches saturation. Accordingly, the ion current is decreased after exceeding its peak value (see FIG. 6).
Meanwhile, if the cylindrical electrodes included within the quadrupole are made shorter, the probability of collisions between the ions passing through the quadrupole and the gas is small even at higher ambient pressures, and a measurement can be realized even in an ambient pressure which is equal to or larger than the predetermined value.
However, the shorter cylindrical electrodes cause a problem of resolution reduction.
As shown in Patent Document 1, there is a conventional gas analyzer which provides a total pressure measurement section (code 11 in Patent Document 1) for measuring a total pressure in the vicinity of the ionizer, so that a partial pressure being a measurement pressure is corrected by using the total pressure in the vicinity of the ionizer.
However, the gas analyzer as described above has a problem that an accurate correction cannot be realized even if a partial pressure is corrected by using the total pressure, unless the total pressure has an accurate measurement pressure used for a correction. In practice, the total pressure which was thus measured stops being proportional to a change of an ambient pressure when the ambient pressure becomes higher than a predetermined pressure (about 1×10−1 to 1 Pa) as described above. In particular, there is a problem that the partial pressure cannot be corrected using the total pressure when the total pressure exceeds its peak value.
Non-patent Document 1: “Feature article: Microminiaturized residual gas analyzer, PressureMaster RGA series” by Toni Ikeda, Horiba Technical Reports, Horiba, Ltd., March 2004, No. 28, pp. 12-15
Patent Document 1: Japanese Unexamined Patent Publication No. 1999-31473